1. Field of the Invention
The present invention relates to a method for producing unsintered cristobalite particles.
2. Description of the Prior Art
Generally, transparent quartz glass is produced by vitrification of silica particles by heat-fusing. The form of heat-fusing includes the Verneuil's method of fusing silica particles with argon-oxygen plasma flame or oxyhydrogen flame, and the vacuum fusion method of heat-fusing silica packed in a container under high vacuum.
Hitherto, natural rock crystal has been used as a material for transparent quartz glass. At present, with an increase in the integration degree of LSI in the field of semiconductor, it is being severely required for raw materials used therefor to have a higher purity, but good-quality natural rock crystal is being exhausted. Studies intended to substitute synthetic high-purity silica for natural rock crystal are now made as a countermeasure therefor.
However, when amorphous synthetic high-purity silica is used as a material in the above method, the quartz glass obtained has a problem of containing many bubbles, because although this silica is of high purity, complete removal of bubbles contained in it is very difficult. This is thought to result from the silica, a material, being porous and short of denseness.
As one of the methods to solve this problem, it is proposed to use amorphous synthetic silica after converted to cristobalite having a dense structure, for example, as disclosed in EP-A2-0173961.
By the above method, transparent quartz glass containing no bubbles can be obtained even with amorphous synthetic silica.
In the conventional method, however, an alkali metal component is added in large amounts when amorphous synthetic silica is converted to cristobalite, so that the alkali metal component remains in considerable amounts even after vitrification, which becomes a problem of quality for high-purity quartz glass. Removal of the alkali component is difficult because it requires prolonged heat-treatment. Also, cristobalite obtained by the conventional method has a form of sintered boy, so that there is a problem that the cristobalite itself is not suited to use as a material for the Verneuil's method.